A method of removing liquid from a sunken floating roof of a liquid storage tank

ABSTRACT

A method for removing liquid from a sunken floating roof of a liquid storage tank. The method comprises the steps of: withdrawing liquid from below the sunken floating roof; and siphoning liquid from above the floating roof to below the floating roof when the liquid level of the tank is below the accumulated liquid above the floating roof.

United States Patent Wagoner et a1.

[54] A METHOD OF REMOVING LIQUID FROM A SUNKEN FLOATING ROOF OF A LIQUIDSTORAGE TANK [72] Inventors: Robert B. Wagoner, Griffith, lnd.;

John S. Kinghorn, Houston, Tex. [73] Assignee: Aerojet-GeneralCorporation- [22] Filed: April 23, 1971 211 Appl. No.: 137,008 r IRelated us. Application Data [62] Division of Ser. No. 15,112, Feb. 27,1970, Pat.

[52] US. Cl. ..l37/l, 137/128, 137/136,

137/143 [51] Int. Cl. ..F04f 10/02 [58] Field of Search .....137/l, 128,136, 143; 220/26 [151 3,701,458 [451 Oct. 31, 1972 [56] References CitedUNITED STATES PATENTS 2,846,109 8/1958 Larsen 220/26 D FOREIGN PATENTSOR APPLICATIONS 1,382,329 11/1964 France ..220/26S PrimaryEraminen-Henry T. Klinksiek V Attorney-Edward O. Ansell and D. GordonAngus r ["57] ABSTRACT A method for removing liquid from a sunkenfloating roof of a liquid storage tank. The method comprises the stepsof: withdrawing liquid from below the sunken floating roof; andsiphoning liquid from above the floating roof to below the floating roofwhen the liquid level of the tank is below the accumulated liquid abovethe floating roof.

9 Claims, 3 Drawing Figures A METHOD OF REMOVING LIQUID FROM A SUNKENFLOATING ROOF OF A LIQUID STORAGE TANK BACKGROUND OF THE INVENTION Thisinvention relates to liquid storage tanks of the type provided with amovable roof or deck which floats upon and is supported by the liquid inthe tank. It particularly relates toa method which will efficiently andautomatically remove liquid from a sunken pan type floating roof withoutthe assistance of personnel enterin g the tank and without structuraldamage to the roof.

The roofs in floating roof installations may sink because, for example:

l. Out-of-roundness of the tank shell due to settlement of thefoundation. The pan binds on the shell and tilts as the liquid levelchanges causing liquid to over-flow into the pan. 2. Improper operationof mixers may throw liquid into the pan.

3. A rapid insertion of gas-laden liquid under the deck may createtilting due to a gas bubble which binds the pan to the tank shellcausing liquid to over-flow the pan. 7

4. Rain water may accumulate in the pan. In short, the roof will sinkwheneverthe weight of the roof plus liquid accumulated in the pan isgreater than the weight of a volume of liquid equal to the volumedisplaced by the roof.

The floating roof frequently has support legs to sup.-

port the roof ,on the floor of the tank. These legs, especially incovered floating roof tanks, are not designed to support any load exceptthe weight of the pan and a few inches of liquid standing above the pan.Since the rim of the pan is about inches high, it is possible for a 15inch liquid level to form on the pan when the pan is on its legs on thefloor of the tank. This amount of liquid will do no harm as long as itis balanced by a counter pressure of liquid underneath the pan. But,when liquid is withdrawn from the tank to allow workers to repair thetank and the sunken roof, then the liquid underneath the pan must beremove, thus resulting in an unbalanced force of 15 inches of liquidabove the pan which cannot normally escape. The economically designedlegs can not support the roof and such liquid and will fail.

In the past two methods have been used to unload product from the pan ofa sunken roof. First, a hot tap may be made into the tank shell througha welded fitting and the product then removed by a suction apparatus.Second, a workman may walk onto the sunken roof with an oxygen mask tomanually remove the drain plug, which is located in the center of theroof for this type of emergency use. Both of these methods arehazardous, costly and require experienced personnel. Thus, a simple,safe, economical method and apparatus for removing the liquid productfrom above the roof is required.

SUMMARY OF THE INVENTION Therefore, one object of this invention is toprovide a method of removing liquid from a sunken pan type floating roofthat is simple, safe, and economical.

Another object is to provide a method of removing liquid from a sunkenpan type floating roof without causing structural damage to the floatingroof.

To attain these and other objectives the present invention provides anovel method to remove the accumulated liquid from the roof panautomatically at the same time the liquid is being removed from belowthe pan, The invention utilizes the theory of an ordinary siphon. Asiphon is a pipe or tube bent to form two legs of unequal length, bywhich a liquid can be transfered to a lower level,-over an intermediateelevation, by atmospheric pressure forcing the liquid up the shorterbranch of the pipe immersed in it, while the excess of weight of theliquid in the longer branch (when once filled) causes a flow. Trappedair or other gas present withinthe pipe will prevent flow of liquidunless there is provision for removing the air.

The present method comprises the steps of: withdrawing liquid from belowthe sunken floating roof; and siphoning liquid from above the floatingroof to below the floating roof when the liquid level in the tank isbelow the accumulated liquid above the floating roof. The rate of liquidflow from below the'floating roof should be equal to or less than therate of liquid flow being siphoned from above the floating roof to belowthe floating roof.

Apparatus to facilitate the above-mentioned liquid removal methodprovides a generally inverted U- shaped siphon drain having first andsecond upstanding leg portions connected together at their respectiveupper ends by a connecting portion; the bottom end of the second portionextends below the roof and the bottom end of the first portion ispositioned above the roof. A check valve is attached to the connectingportion to allow trapped air to escape from the pipe only in avertically upward direction.

DESCRIPTION OF THE DRAWING The invention, both as to its organizationand method of operation, taken with further objects and adlustrating thesiphon drain of the instant invention and FIG. 3 is an enlarged planview, partially broken away, of the check valve of the instantinvention.

DISCLOSURE OF THE INVENTION Referring now to the drawings, and moreparticularly to FIG. 1, there is illustrated a closed storage tank ofthe floating roof type identified generally by reference numeral 10. Thestorage tank 10 serves to store liquid, for example, petroleum products,which is fed to and withdrawn from the tank 10 through shell nozzle 14which communicates with the lower portion of the tank 10 The tank 10includes a circular floor 12 which is I located on the ground and agenerally cylindrical shell 16 extending upwardly from the periphery ofthe floor 12. The shell 16 supports the edge of a fixed roof 18 that hasan upwardly extending conical shape. In addipose of maintaining anyvapor adjacent to the liquid, a

floating roof 28 floats on and extends over substantially the entiresurface of the liquid. As is known to those skilled in the art, thefloating roof 28 moves vertically within the storage tank as the levelof the liquid within the tank 10 changes. To prevent vapor loss, aconventional resilient sealing arrangement 30 is supported from theperiphery of floating roof 28 so as to frictionally engage the shell 16,irrespective of the position of the floating roof 28 within the tank 10.The floating roof 28 is suitably apertured, as indicated by referencenumeral 32, to accommodate support and guidance columns and is suitablyapertured, as in dicated by reference numeral 34, to accommodate theladder 22. Conventional sealing arrangements (not shown) are provided onthe floating roof 28 to respectively coact with the columns 20 andladder 22 for purpose of preventing vapor loss.

. The specific construction of the floating roof 28 in general isimmaterial to the instant invention. The' floating roof 28 is a simplepan, having a rim 29 about 15 inches high, that floats on the surface ofthe liquid product. Supporting the floating roof 28 above the floor 12during the start-up, cleaning operations, and repair operations aresupport means 41. Support means 41 are affixed to roof 28 throughsleeves 33 and are economically designed not to support any load exceptthe weight of the roof and a few inches of liquid standing above thepan.

For reasons previously discussed liquid will occasionally accumulate onthe roof causing same to sink to the bottom of the tank. It then becomesnecessary to remove all the liquid from the tank for repair purposes.After sinking, the floating. roof, including the liquid containedthereabove, is supported on the floor of the tank by support means 41.Support means 41 will support the sunken roof as long as it is balancedby a counter pressure of liquid in the bottom of the tank underneath thepan. But when liquid is withdrawnfrom the tank to allow workers torepair the tank and sunken pan, then the liquid underneath the roof mustbe removed, thus resulting in an unbalanced force of about 15 inches ofliquid above the pan which cannot normally escape. This causes the legs41 to fail.

The present invention provides a method of removing the liquid from thetank without increasing the resultant downward force on the roofsupporting means. Liquid is withdrawn from the tank at the standardwithdrawal rate through the shell nozzle 14, which is below the roof,until the level reaches down to approximately 1 inch below the top ofthe rim 29 of the floating roof. At this time the withdrawal rate isdecreased and liquid is siphoned from above the floating roof to belowthe floating roof until the roof is substantially cleared of liquid. Thewithdrawal rate is decreased in order to prevent liquid from beingwithdrawn from below the floating roof at a greater rate than liquidbeing siphoned from above to below.

the floating roof. If a 6 inch standard pipe siphon is employed thewithdrawal rate is decreased to approximately 220 barrels/hr. Should adifferent size siphon be used then the withdrawal rate has to be changedto make certain that the withdrawal rate is equal to or less than therate in which liquid is siphoned from above to below the floating roof.By so regulating the withdrawal rate and siphon rate, structural damageto the floating roof, caused by an unbalanced downward force ofaccumulated liquid in the pan, is eliminated. The withdrawal rate may beincreased to the standard withdrawal rate alter the accumulated liquidis removed from abovethe roof.

- Referring now to FIGS. 1 and'2, the preferred em- I 'bodiment of thepresent invention includes apparatus having a siphon drain 50 fabricatedfrom 6 inch standard pipe and mitered at two joints to form an invertedU-shaped pipe., Siphon drain 50 has-first and second upstanding legportions 51 and 52 respectively connected together at their respectiveupper ends by a connecting portion 53. The bottom end of leg'portion 52extends through and below floating roof 28. Leg portion 51 is positionedabove floating roof 28. Leg portion 51 is positioned above floating roof28 with the bottom end being about 2 inches above the deck of thefloating roof to enable the siphon drain to flow at full capacity. Itshould be noted that leg portion 51 is positioned above nozzle 14thereby allowing liquid exiting from leg portion 52 to be withdrawnthrough nozzle 14. Support means 41 insure that leg portion 51 will notfall below nozzle 14.

A one-half inch check valve is attached to connecting portion 53 in asuitable manner, as by welding, so as to permit fluid communication withsiphon drain 50. As seen in FIG. 3 check valve 60 includes a cylindricalbody 62 having open top and bottom ends 63 and 64 respectively.Extending inwardly about the inner periphery of body 62 is horizontalmember 65 having an aperture 66 therethrough. Positioned below member 65is diametrically extending cross bar 67 having an aperture 68therethrough. Guide pin 69 passes through apertures 66 and 68 and hasnut means 70 and 71 at its respective ends. Nut means 71, being largerin diameter thanaperture 68, prevents the lower end of guide pin 69 frompassing completely through aperture 68. Washers 72 and 73 prevent theupper end of guide pin 69 from passing through aperture 66. Washer 72 isrubber thereby effecting a seal closing off aperture 66 when valve 60 isin the closed position as illustrated in FIG. 3. As the floating roof 28sinks, air and liquid are forced upwardly through aperture 66 liftingguide pin 69 and washer 72 and allowing communication between siphondrain 50 and atmosphere. Check valve 60 prevents air pockets fromdeveloping in the siphon drain which would prevent the siphon fromoperating.

In operation, as the floating roof 28 accidentally sinks an air pocketis developed in the top of the siphon drain 50 below check valve 60. Theair is trapped since liquid begins to enter into both ends of legportions 51 and 52 of the siphon drain; however, check valve 60 quicklyallows for this trapped air to escape, therefore making the siphon drainoperable immediately after it has sunk below the surface of the liquid.Eventually, floating roof 28 will land on its support means 41 on thetank floor 12. Liquid is then Withdrawn from the tank at the standardwithdrawal rate through shell nozzle 14, until the liquid level reachesdown to about 1 inch below the top of the floating roof rim 29. Liquidpasses from above the roof to below the roof through seals 30, apertures32 and sleeves 33. At this time the withdrawal rate is decreased toapproximately 220 barrels/hr. until the roof is cleared of liquid.Liquid from above the roof automatically flows up leg portion 51, acrossconnecting portion 53 and down leg portion 52 to below the roof at arate equal to or less than the withdrawal rate. The approximate timerequired for unloading the roof would be 8 hours for a 100 foot diametertank and 31 hours for a 200 foot diameter tank. This is a small amountof time compared to the amount of time spent in the past repairingdamage to a roof that has been overloaded.

As can be appreciated by those skilled in the art, the present inventionprovides a novel method and apparatus for removing liquid from a sunkenfloating roof of a liquid storage tank. The invention eliminates theneed for personnel entering the tank or damaging the tank shell orfloating roof. The method and apparatus for carrying out the method, aresafe and may be economically installed on new and existing floating roofstorage tanks.

While the embodiment described herein is at present considered to bepreferred, it is understood that various modifications and improvementsmay be made therein, and it is intended to cover in the appended claimsall such modifications and improvements as fall within the true spiritand scope of the invention.

What is claim is:

l. A method of removing liquid from a storage tank having a sunkenfloating roof, comprising the steps of:

a. withdrawing liquid from the tank below the floating roof; and

b. siphoning liquid from above the floating roof to below the floatingroof.

2. The method of claim 1 further characterized by siphoning the liquidfrom above the floating roof only when the liquid level in the tank isbelow the accumulated liquid above the floating roof.

3. A method of removing liquid from a storage tank with a sunkenfloating roof having bottom support legs, comprising the steps of:

a. withdrawing liquid from the tank below the floating roof; andsimultaneously b. removing liquid from above the floating roof to belowthe floating roof so as to reduce the pressure exerted upon the supportlegs as liquid is withdrawn from below the floating roof."

4. The method of claim 3 further characterized by removing liquid fromabove the floating roof to below the floating roof by siphoning means.

5. The method of claim 4 further characterized by siphoning the liquidfrom above the floating roof only when the liquid level in the tank isbelow the accumulated liquid above the floating roof.

6. The method of claim 5 further characterized by reducing the rate ofliquid flow from below the floating roof to equal to or less than therate of liquid flow being siphoned from above the floating roof to belowthe floating roof.

7. A method of removing liquid from a storage tank having a sunkenfloating roof, comprising the steps of:

a. withdrawing liquid from the tank through a nozzle

1. A method of removing liquid from a storage tank having a sunkenfloating roof, comprising the steps of: a. withdrawing liquid from thetank below the floating roof; and b. siphoning liquid from above thefloating roof to below the floating roof.
 2. The method of claim 1further characterized by siphoning the liquid from above the floatingroof only when the liquid level in the tank is below the accumulatedliquid above the floating roof.
 3. A method of removing liquid from astorage tank with a sunken floating roof having bottom support legs,comprising the steps of: a. withdrawing liquid from the tank below thefloating roof; and simultaneously b. removing liquid from above thefloating roof to below the floating roof so as to reduce the pressureexerted upon the support legs as liquid is withdrawn from below thefloating roof.
 4. The method of claim 3 further characterized byremoving liquid from above the floating roof to below the floating roofby siphoning means.
 5. The method of claim 4 further characterized bysiphoning the liquid from above the floating roof only when the liquidlevel in the tank is below the accumulated liquid above the floatingroof.
 6. The method of claim 5 further characterized by reducing therate of liquid flow from below the floating roof to equal to or lessthan the rate of liquid flow being siphoned from above the floating roofto below the floating roof.
 7. A method of removing liquid from astorage tank having a sunken floating roof, comprising the steps of: a.withdrawing liquid from the tank through a nozzle positioned below theroof at a relatively high flow rate until the liquid level of the tankis below the accumulated liquid above the roof; b. decreasing the flowrate from the nozzle to a predetermined rate; and simultaneously c.siphoning the accumulated liquid from above the roof to below the roofat said predetermined rate.
 8. The method of claim 7 furthercharacterized by supporting the roof above the floor of the tank toinsure that the roof is always above the nozzle.
 9. The method of claim7 further characterized by increasing the rate of flow from the nozzlewhen the liquid has been removed from above the floating roof.